We investigated the functional activity of the influenza polymerase PB2 protein and initiated attempts to establish persistent expression of PB2 polymerase functions by transfecting cloned PB2 DNA into permissive cells. Functional expression of the polymerase genes should provide a complementation system for growth of influenza virus mutants with mutations affecting these genes. This type of growth complementation should be useful for isolating naturally occurring or laboratory engineered mutants containing defects in a predetermined polymerase gene. Several PB2 DNA recombinants were used to construct plasmid vectors for expression of PB2 in permissive cells. For lytic infection of primate cells SV40-PB2 recombinant DNA was successfully propagated in the presence of a helper SV40 early function mutant. Dot-blotting and "Northern" blotting mRNA analyses are currently being carried out to detemrine whether the PB2 protein encoded by the recombinanat is functionally active. For persistent expression we employed a cloned mutant dihydrofolate reductase (DHFR) gene as a selectable marker. These studies have yielded several cell populations that survivied drug selection. These cells are being analyzed for their ability to complement influenza mutants defective in PB2 function.